Bob,
Of the 100 or so Unionids or the smaller fingernail clams that live(d) in
Indiana, any speculation would be fairly difficult.  Usually though, you
will not find too many juvenile Unionids in the mid-west streams.  Two
possible choices would be either the introduced Corbicula cf. manilensis
which is not a Unionid but is more oval with many concentric very close
growth rings, or the smallest of the Unionids, Truncilla donaciformis, which
would be more elongate than round. The Corbiculas can range from green to
black, but mostly they are a pale apple green uniform color.
Marion Havlik who is on the list has done a lot of work on the aging of
Unionids by growth rings and she can address that topic.  The mollusks will
about this time of year start burrowing down and resting until the next
season, so the growth lines on Unionds are fairly good indicator.
A fun old book if you can find it is Richard Ellsworth Call  M.D. An
Illustrated Descriptive Catalogue of the Mollusca of Indiana.  My copy does
not have the publication date but I think it is 1900, and of  course the
nomenclature has changed, but it is a nice book to look at.  No Corbicula
though in that book.
Kevin Cummings's Midwest Molluscs is on line at
http://www.inhs.uiuc.edu/cbd/collections/mollusk/fieldguide.html and is a
fantastic resource.
I was surprised to find how few Unionds there were in some middle Illinois
streams on a trip there a couple of weeks ago.  They had record drought in
that area and very, very few dead shells were evident.

Alan Gettleman
Merritt Island, FL
----- Original Message -----
From: "Bob Dayle" <[log in to unmask]>
To: <[log in to unmask]>
Sent: Sunday, October 09, 2005 5:49 PM
Subject: Freshwater clam question...


> I was wading in the local creek (central Indiana) when I noted a single
valve in
> the foot-deep water. Hoping to stir up its mate, I came up, instead, with
a
> live clam. At a mere 3/4-inch across, it was rather small,... to someone
who
> has seen three foot wide Tridachna gigas in situ.
>
> Since it is illegal in this state to disturb or molest ANY mollusk, I
tossed it
> back after noting that the valves were dark brown at the hinge but quickly
gave
> way to an algae-green color other the rest of the shell. I estimated about
15
> "growth rings" on it.
>
> The questions are two: 1.) Does anyone care to guess what variety of
plecypod
> this is?, and 2.) What time interval is associated with the growth
marks,...
> annual, lunar, sesquiquadrate aspects to the synodic period of Venus,.. or
> what?!
>
> Bob Dayle
>
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